1. Field of the Invention
The present invention relates to a power supply apparatus provided with an auxiliary power supply in addition to a power supply that uses externally-supplied power in a steady state. The auxiliary power supply has a power storage device and a power supply circuit that uses electric power from the power storage device as a power source. More particularly, the present invention relates to, although the example is not intended to limit the invention, a power supply apparatus that supplies electric power stored in a power storage device to a direct current (DC) load when a large amount of power is required by entire equipment, thereby leveling power consumption by the equipment so as not to exceed an amount of available commercial alternating current (AC) power. Examples applications of the invention include a power supply apparatus, a printer, a copier, and a facsimile that supplies backup power when power supply from the outside is disrupted, supplies high-load power exceeding a maximum capacity of the externally-supplied power, or levels power consumption of the externally-supplied power.
2. Description of the Related Art
In recent years, a copier, a printer, a facsimile each utilizing an electrophotographic process, and a multifunction product having two or more functions thereof have been increased in versatility, which has increased the complexity of the products and thereby increasing maximum power consumption.
Power supply to a fixing heater has been increased in amount to reduce factors inherent to an image forming apparatus, such as a warm-up time of a fixing unit or a suspension time due to a drop in the temperature of the fixing heater during a printing or copying operation, or to reduce a waiting time of a user. However, electric power available from a normal power supply line is limited in its amount. This constitutes a large restriction on apparatus design. Japanese Patent Application Laid-open No. 2004-236492 describes a power supply apparatus and an image forming apparatus devised for not exceeding a maximum available power of a power supply line. To achieve this, power consumption is predicted. When the thus-predicted power consumption exceeds a maximum available power of a main power supply, the apparatus supplies power to some of loads while switching power source therefor between the main power supply and an auxiliary power supply using a switching circuit.
Japanese Patent Application Laid-open No. 2005-221674 describes an image forming apparatus that uses a constant voltage power supply circuit as an auxiliary power supply, and sets an output voltage therefrom higher than an output voltage from a main power supply. A first diode for preventing backflow of power to the main power supply is provided on a power supply line from the main power supply to the load, and a switch or a second diode is provided on the power supply line at a point between the first diode and the load. The output voltage from the auxiliary power supply is applied to the load via the first diode, and the switch or the second diode. In a period in which the output voltage from the auxiliary power supply is higher than the output voltage from the main power supply, power is supplied to the load only from the auxiliary power supply.
However, according to the conventional techniques, the power storage device uses a constant-voltage power supply as the power output circuit; that is, as the power supply circuit for the load. Hence, when power supply source is switched between an AC/DC power supply (main power supply), which is a constant-voltage power supply, and an auxiliary power supply, which is also a constant-voltage power supply, voltage undesirably fluctuates due to a difference between two output voltages from the constant-voltage power supplies. The irregular rotation of the motor can result in an anomalous image formed by the image forming apparatus. For example, when the apparatus is a color image forming apparatus, the irregular rotation can cause color misregistration.
Information on Prior Application
The present inventors have proposed a power supply apparatus that supplies electric power to a load. The apparatus includes: a first power supply that supplies a constant voltage by using externally-supplied electric power as a power source; a second power supply that has a power storage device and supplies a constant current by using electric power of the storage device as a power source; a circuit unit that connects an output from the first power supply and an output from the second power supply to the load in parallel, thereby simultaneously supplying electric power from both the first power supply and the second power supply to the load; a current detector that detects a load current value, which is a value of an electric current supplied to the load; and a control unit that controls an output current from the first power supply to within an indicated maximum value, and controls an output current from the second power supply to a value obtained by subtracting the indicated maximum value from the load current value detected by the current detector (Japanese Patent Application Laid-Open No. 2006-49221, filed on Feb. 24, 2006). According to the technique, power is supplied to the load from the first and second power supplies simultaneously. This eliminates the need of switching between the two power supplies, thereby suppressing voltage fluctuation due to the switching.
In the apparatus, detection of the current supplied to the load and indication of a current to be supplied to the second power supply are implemented via analog circuit units. When an abnormality occurs in the circuit units, power consumption by the power supply apparatus can increase to exceed a rated power, and cause a voltage drop or the like of commercial power source from which power is supplied to the power supply apparatus.